1. Field of the Invention
This invention generally relates to laser scanning systems for reading bar code symbols or similar indicia and, more particularly, to a laser imaging system for generating a laser beam scan pattern which extends in a two dimensional pattern over the symbols to be read, and a digital processing system for storing, manipulating, and analyzing the scanned image.
2. Description of the Related Art
Various optical readers and optical scanning systems have been developed heretofore for reading bar code symbol appearing on a label or on the surface of an article. The bar code symbol itself is a coded pattern of indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light-reflecting characteristics. A number of different bar code standards or symbologies exist. These symbologies include UPC/EAN, Code 128, Codabar, and interleaved 2 of 5. The readers and scanning systems electro-optically decode the symbol to multiple alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. Such characters are typically represented in digital form as an input to a data processing system for applications in point-of-sale processing, inventory control, and the like Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,409,470 and 4,460,120, all of which have been assigned to the same assignee as the instant application
As disclosed in some of the above patents, one embodiment of such a scanning system resides, inter alia, in emitting a laser light beam from a hand-held, portable laser scanning head supported by a user, and aiming the head, and more particularly, the laser light beam, at a symbol to be read. The scanner functions by repetitively scanning the laser beam in a line across the symbol. A portion of the reflected laser light which is reflected off the symbol is detected, and electronic circuitry or software decodes the electrical signal into a digital representation of the data represented by the symbol scanned.
More specifically, a scanner includes a light source such as a gas laser or semiconductor laser that generates a light beam. The use of semiconductor devices as the light source in scanner systems is especially desirable because of their small size, low cost and low power requirements. The light beam is optically modified, typically by a lens, to form a beam spot of a certain size. It is preferred that the beam spot size be approximately the same as the minimum width between regions of different light reflectivity, i.e., the bars and spaces of the symbol. The relative size of the bars and spaces is determined by the type of coding used, as is the actual size of the bars and spaces. The number of characters per inch represented by the bar code symbol is referred to as the density of the symbol.
The light beam is directed by the lens or similar optical components along a light path toward a target that includes a bar code symbol on the surface. A scanning component is also disposed in the light path. The scanning component may either sweep the beam spot across the symbol and trace a scan line across and past the symbol, or scan the field of view of the scanner or do both. A scanner also includes a sensor or photodetector. The photodetector has a field of view which extends across and slightly past the symbol and functions to detect light reflected from the symbol. The analog electrical signal from the photodetector is first typically converted into a pulse width modulated digital signal, with the widths corresponding to the physical widths of the bars and spaces. Such a signal is then decoded according to the specific symbology into a binary representation of the data encoded in the symbol, and to the alphanumeric characters so represented.
Laser scanners are not the only type of optical instrument capable of reading bar code symbols. Another type of optical reader is one which is operative being placed by the user in direct contact with the symbol to be read. Such readers typically incorporate detectors based upon charge coupled device (CCD) technology in which the size of the detector is larger than or substantially the same as the symbol to be read. Such scanners are lightweight and easy to use, but require substantially direct contact or placement of the reader on the symbol to enable the symbol be read. Such contact reading is a preferred mode of operation for some applications or as a matter of personal preference by the user. Contact or near contact reading may also be implemented in an appropriately designed laser scanner.
With respect to the scanning pattern generated by laser scanners, it has previously been known to generate scan patterns of mutually parallel scan lines extending along one or two directions, especially for fixed, stationary, or table-top type scanners. Omni-directional scan patterns of intersecting scan lines, and even curvilinear scan patterns, for superposition over the symbol to be read, are known in the prior art. The purpose of such scanning patterns is to insure that, no matter what the angular orientation of a symbol might be, within predetermined limits, at least one of the scan lines or part of the pattern will be scanned over the entire length of the respective symbol. A drawback of such an approach is that such patterns are typically generated by highly complex and expensive optical and mechanical systems.
U.S. patent application Ser. No. 944,848 now U.S. Pat. No. 4,816,661 describes a hand-held laser scanner that generates a scan pattern of mutually parallel scan lines extending linearly across each symbol. The scan lines are arranged over the height of the symbol. At least one of the scan lines sweeps across each symbol along one sweep direction, whereas, at least another of the scan lines sweeps across each symbol along an opposite sweep direction countercurrent to said one sweep direction, thereby forming a bidirectional scan in which a respective symbol can be read by the a scan line no matter whether the symbol is oriented in the correct left-right direction, or is upside down.
In either hand-held or stationary scanners, with the scan being generated by mechanically oscillating or moving a light source or a mirror, the direction of each scan is fixed relative to the reader housing. If there is only one scan line, and the bar code symbol is located at an angle to this linear scan of the laser beam, then the user of a hand-held unit must twist the unit to align it with the angle of the bar code. Or, the operator of a stationary unit must twist the product to retry, hoping to align the symbol with one of the several scan lines. When a hand-held reader unit is being used at a retail check-out counter, the objects having bar codes to be read will be randomly oriented, and the items will be of many different sizes and shapes. Properly positioning the hand-held reader unit for reading bar codes thus becomes an awkward task. On the other hand, scanners generating several scan paths at angles to one another still often require the operator to try several passes of the product until a valid read is obtained. Multiple-scan readers produce only a few fixed paths for scan lines, and if none of the paths are correct the operator must make additional passes until a code-recognition signal is produced.
In order to provide more information in the bar code symbols, and to allow the symbols to be smaller or more compactly shaped, new bar code standards have been adopted. One of these new code standards, Code-49, uses a more complex but efficient character set, and also introduces a "two-dimensional" feature so more data is contained in the symbol; this is accomplished by stacking rows of characters vertically instead of extending the bars horizontally. That is, there may be two or more rows of bar and space patterns up to eight, instead of only one row.
U.S. Pat. No. 4,794,239 is hereby incorporated by reference as describing such a bar code structure. A one-dimensional single-line scan, as ordinarily the case for hand-held readers, where the laser beam is swept back and forth across a narrow arc, has disadvantages in reading these two dimensional bar codes; that is, the reader must be aimed at each row, individually. Likewise, the multiple-scan-line readers produce a number of scan lines at an angle to one another so these are not suitable for recognizing Code-49 type of two-dimensional symbols. Prior to the present invention, there has not been a laser scanner that is particularly suitable for reading two dimensional bar code symbols or other indicia.